Miniaturized tri-notched wideband bandpass filter with ultrawide upper stopband suppression

Stepped impedance resonator (SIR) and its derivative resonators are widely used in the design of microwave filters. However, many spurious modes will be introduced into the stopband, resulting in lower upper stopband suppression and performance degradation. Based on the principle of slit line, a method to enhance the upper stopband suppression is proposed and verified by a miniaturized tri-notched wideband bandpass filter based on stub loaded ring resonator (SLRR) and shorted-stub loaded SIR (SSLSIR). The wideband is formed by coupling SLRR and interdigital lines, which has a rectangular DGS on the back of the substrate. Three notched bands with controllable positions in the passband can be produced by inserting two different SSLSIRs inside and outside the SLRR. Four slit lines are loaded on the low impedance stub of SSLSIR to adjust the high-order modes close to the transmission zeros (TZs). The operating frequency of the filter is 2.2-7.6 GHz, and the three notched bands are located at 2.97 GHz, 5.75 GHz and 6.46 GHz, respectively. The measurement results show that the - 20 dB enhanced upper stopband of the filter can reach 32 GHz, which proves that the filter has the characteristic of ultrawide upper stopband suppression while keeping the miniaturization.